Insulator for radio frequency currents



Jan. 22, 1929. 1,700,066

A; E. MARSHALL INSULATOR FOR RADIO FREQUENCY CURRENTS Filed July 17,1924 w w di Patented Jan. 22, 1929.

UNITED-STATES PATENT OFFICE.

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Application fled my 17, 1924. Serial No. 720,597.

Insulation against passage of radio frequency currents and especiallythose of very high frequency is attended with great 'difliculties. Byradio frequency currents are, meant currents having a frequency of saybetween 10,000 to 300,000,000 cycles .per second. This is particularlymarked as the frequency increases and goes above 1,000,000, so that1nsulators which are suitable for use w th OIdlnary alternating powertransmission currents or currents of telephonic frequency are not ofnecessity effective when subjected to the high frequencies named.v Thisis due to the fact that the power factor losses are usually higher withthe high frequency currents than with those of low frequency. Generallyspeaking, power factor losses with an alternating current increasewith'the frequency, with the dielectric constant and with the phasedifference.

The, dielectric constant is the only one of these factors which comesinto play to any appreciable extent in low frequencycurrents, and forthis reason values of insulatlng materials in radio frequency work havebeen generally evaluated by the dielectric-constant. Measurement showsthat many of the insulating materials now in common use which have lowdielectric constants and which thus appear suitable for radio work, havelar e phase angle difference, and that the su stances which have lowphase differences have high dielectric constants and are thus apparentlyunfitted for radio work, An illustration of this is glass, which isgenerally regarded as having high dielectric constants and thereforeunsuitable for radio insulation, especially at the higher frequencies.'This belief has been general in the industry and by those skilled inthe art.

I have discovered however that certain glasses possess properties whichrender them especially suited for high frequency radio work, in thatwhile their dielectric constants are not unduly high, they possess asmall phase angle difference which only increases slowly with increaseof temperature. Glasses possessing this property are generally speakingthose containing a high percentage of silica, a low percentage ofalkali, the alkali percentage being preferably in the form of potashinstead of soda, and which can be melted into a true homogeneousnon-crystalline mass. Boric acid is preferably also present as tendingto the complete fusion and as producing non-hydroscopic and chemicalstable glass which tends to preserve a smooth surface under weatheringconditions. Glass of the general composition above stated also has theadvantage that the principal elements in the composition, that is tosay, boric oxide and silica, both possess electrO-negativecharacteristics. and that the ions of the alkali that are introduced inthe preferred composition given are relatively immobile.

A glass suitable for the purpose above indicated is disclosed in UnitedStates Letters Patent No. 1,304,623, dated May27th, 1919, to Eugene C.Sullivan and William O. Taylor, and particularly composition B thereof.The glass in question contains 80.9% silica; 12.9% boric oxide; 4.4sodium oxide and 1.4 aluminum oxide,the alumina tending to stability andfusibility. As suggested, the soda can advantageously be replaced bypotash.

I have represented in the accompanying drawings forming part of thisapplication an antenna insulator which is one of the numerous devices towhich the invention here disclosed may be applied. In such specific formthe chemical durability of the glass of the insulator and itsnon-hydroscopic charac-, ter tends to prevent the formation of aconducting film of dirt or moisture, while the phase angle differencepossessed by the glass in question, to-wit ar at 500,000 cycles, reducesto a minimum the power loss of the energy in the antenna. If on atransmitting antenna, which has a considerable power input, the smallpower loss in the insulator reduces to a minimum the increase oftemperature attendant on absorption of power, thereby reducing to aminimum the danger of fracture or breakage from sudden changes oftemperature and also keeps at a minlmum the increased power loss due toincrease in temperature of the glass itself.

Having thus described my invention what I claim is c 1. In a systemcarrying radio frequency currents, the combination with a part chargedwith such currents, of an insulator 7 charged with such currents, of aninsulator therefor composed of a homogeneous mixture therefor composedof a glass having a high having a high silica content, and containing 10silica content, a low alkali content and conboric oxide and a smallamount of potash.

5 taining boric oxide. In testimony whereof I hereunto afiix my 2. In asystem carrying radio frequency signature. currents, the combinationwith a part ALBERT EDWARD MARSHALL.

